The invention relates to improving the performance of USSD (Unstructured Supplementary Service Data) transfer in a cellular communications system, such as GSM (Global System for Mobile Communication).
The user of a mobile station (MS) can use USSD to give instructions to the supporting PLMN (Public Land based Mobile Network). For example, incoming calls can be routed to number 123456 by dialling *21*#123456#. USSD is also one of the mechanisms for implementing new services. USSD allows an MS and a service application to communicate with each other by character strings, in a way which is transparent to the MS and to the intermediate network elements. USSD can be used as a narrow-band bearer for over-the-air (OTA) and value-added services (VAS) applications. With respect to a more detailed description of the USSD, reference is made to the following ETSI GSM recommendations: GSM 02.90: European digital cellular telecommunications system (Phase 2); Stage 1 description of Unstructured Supplementary Service Data (USSD), GSM 03.90: Digital cellular telecommunications system (Phase 2); Unstructured Supplementary Service Data (USSD)xe2x80x94Stage 2, and GSM 04.90: European digital cellular telecommunications system (Phase 2); Unstructured Supplementary Service Data (USSD)xe2x80x94Stage 3. USSD requests, notifications and responses contain a USSD string, an alphabet indicator and a language indicator, as defined in GSM 03.38.
USSD signalling may be initiated by the mobile station or by the network. Phase 1 supports only MS-initiated USSD. Network-initiated USSD service requires that all parts of the mobile communications system be at least phase 2 systems. The mobile communications network may at any time send a USSD message to a mobile station MS registered with the network in order to transmit information to the subscriber. This operation may be either a request (asking the MS to provide information) or a notification (requiring no information to be provided by the MS). No prior provisioning of USSD is required, although provisioning of services which make use of USSD may be required.
According to the above ETSI recommendations, USSD signalling takes place between an MS and an MSC/VLR (Mobile services Switching Centre/Visitor Location Register) or HLR (Home Location Register). USSD supports a maximum of 160 bytes of usher data per message. (The upper limit can be less than 160 bytes depending on the underlying protocol layers.) Unlike SMS (Short Message Service), USSD has no store-and-forward functionality: mobile-terminated USSD messages are delivered to the MS immediately, or the delivery fails (e.g. because the MS is unreachable).
For the purposes of this application, a GSM-type mobile station has two modes: a call mode and an idle mode. A mobile station is in call mode if and only if it is xe2x80x9cin a callxe2x80x9d, which state is defined in the GSM recommendation 02.30.
According to the above ETSI recommendations, USSO transfer takes place on two different channels depending on whether or not the MS is in call mode or idle mode. In call modes, Fast Associated Control Channel (FACCH) is used. In idle mode, Slow Dedicated Control Channel (SDCCH) is used.
The speed of the FACCH channel is approximately 140 bytes per second and that of the SDCCH channel approximately 83 bytes per second. Thus, even in idle mode, any USSD message can be delivered in less than two seconds.
It is conceivable that the use of USSD for implementing value-added and over-the-air services will increase. In this case, especially if multiple consecutive USSD messages are needed, the slow transfer speed off the SDCCH channel could be seen as a problem. (It should be noted that for keeping the description compact, FACCH is used as a synonym for the fast channel, and SDCCH is used as a synonym for the slow channel. However, FACCH and SDCCH are terms used in the GSM system and its derivatives, but these terms are not necessarily used in future cellular systems.)
Accordingly, it is an object of the present invention to study whether the speed of USSO in idle mode can be improved, and if yes, to provide a method and equipment for improving the speed of USSD in idle mode. The object is achieved with a method and equipment which are characterized by what is disclosed in the attached independent claims. Preferred embodiments are disclosed in the attached dependent claims.
A straightforward way of improving the speed of USSD transfer would be to specify that all USSD traffic takes place on the fast FACCH channel. This would, however, require changes in existing standardisation. Also, FACCH is not a dedicated channel, but an associated one, which means that it is implemented by stealing bits from the speech channel, if one exists. If such bit stealing is allowed to go on for long periods of time, it will degrade speech quality to some extent.
The invention is based on locating the problem and finding a solution for it. The solution is based on the idea that the amount of USSD data is determined, and if the amount exceeds (or is likely to exceed) a predetermined threshold (i.e. for lengthy USSD transmissions), the faster FACCH channel is activated by directing the MS into call mode. A simple way of accomplishing this is performing an unsuccessful call attempt.
Thus it can also be said that the invention is based on a novel interpretation of call mode, as specified in the above-referenced GSM recommendation 02.30, wherein call mode is defined as follows: A mobile station is in a call from the time that signalling related to the establishment or attempted establishment of a mobile originated or mobile terminated call commences, and before the call or call attempt ends, and (if applicable), the mobile equipment has stopped generating tones related to this call to the user. Fooling the MS into call mode (by performing an unsuccessful call attempt) activates the faster FACCH channel for USSD transfer. However, it should be noted that the mobile station is xe2x80x9cin a callxe2x80x9d as defined by ETSI GSM 02.30, whereby the invention requires no deviations from existing standards. Thus the method and equipment according to the invention solve the problem of the prior art USSD transfer in an elegant manner.
The invention is also based on determining the amount of USSD data (i.e. the length of USSD transmission) and using the FACCH channel only for lengthy USSD transmissions. Performing an unsuccessful call attempt for short USSD transmissions would create unnecessary signalling load, which is especially harmful at the air interface. This signalling load must be balanced against the savings in time brought about by the mechanism of the invention. For example, FACCH could be used only if using i saves at least one second. Because FACCH transfer is approximately 1.7 times faster than SDCCH transfer, it saves about 40% of the time needed by the SDCCH transfer. If it is required that at least one second must be saved, a minimum length for a USSD message would be 250 bytes. This exceeds the length of a single USSD message. In other words, performing the unsuccessful call attempt is useful only with multiple consecutive USSD messages (assuming 83 and 140 bytes per second for SDCCH and FACCH, respectively, and a minimum saving of one second). Of course, it must be remembered that initiating the unsuccessful call attempt wastes a fraction of a second. Thus the logic for initiating the unsuccessful call attempt should be placed at the top of the protocol stack, i.e. in the application layer. Otherwise the logic will not know that multiple USSD messages are needed.